1. Field of the Invention
The present invention relates to the field of synchronizing time clocks between distributed network elements, and more particularly, to adjusting packet time stamps by measuring packet residence times in network elements without access to real-time clocks.
2. Description of Related Art
Synchronizing the elements that make up a network is fundamentally important to achieving good network performance. At a basic level, synchronizing distributed clocks requires distributing timing information from a master clock to a number of slave clocks throughout the network. A number of schemes are employed in practice. For example, each network element may include its own real-time clock; synchronized to a master reference via synchronization signals. Dedicated electronic circuits may be used to generate and distribute these master clock synchronization signals to assure sufficiently high frequency accuracy and stability, and sufficiently low phase noise. For example, the IRIG-B protocol employs dedicated coaxial cables and clock drivers that distribute the clock independently of any network data connections and can achieve synchronization accuracies of a few microseconds.
A drawback of such a distribution scheme is that the dedicated clock management and distribution hardware performs only this individual function and consumes a significant fraction of the resources and cost of running a network. This has driven the industry toward timing synchronization methods that employ shared-resource packet-based Ethernet transport mechanisms for the synchronization of network elements, such as Network Time Protocol (NTP). Such packet-based synchronization schemes eliminate the need for expensive precision oscillators or GPS receiving circuits at multiple network nodes and further allow for sharing of hardware resources because timing packets and data packets can share the same physical network. But Ethernet is an inherently asynchronous protocol, posing significant challenges to using it as a basis for precise timing control. Indeed, NTP is prone to traffic-dependent latencies and timing jitter that tend to limit its accuracy to several milliseconds.
The Precision Time Protocol (PTP) IEEE-1588 standard has emerged as a protocol addressing many of the concerns associated with packet-based time synchronization of network elements. PTP addresses the time-transfer latency that arises as time-packet and data-packet traffic moves through the hubs, switches, cables and other hardware that makes up the network. Time Stamping Units, or TSUs, are employed between the Ethernet Media Access Control (MAC) or similar and the physical layer (PHY) transceiver to detect both the arrival and departure of timing packets and to mark them with a precise time stamp. One possible implementation of a PTP protocol has been developed by Semtech Corporation and is known as “ToPSync.” ToPSync employs a master clock timing reference that is distributed to multiple slave clocks associated with various network nodes. A packet is time stamped and sent from the master clock to a slave clock. In turn, the slave clock sends time-stamped packets back to the master. A clock recovery algorithm recreates the master clock time base to synthesize the synchronization signals that are distributed throughout the network and filters out most of the noise and differential propagation delay inherent in the transport network. The IEEE-1588 PTP is capable of synchronizing both frequency and phase and thus can support both frequency-division duplex (FDD) and time-division duplex (TDD) systems. Typical operation involves packet “triplets” comprising a sync message sent from the master clock to a slave, followed by a delay request message from the slave to the master, and then a delay response message back from the master to the slave. Alignment of frequency requires only the sync broadcast, but alignment of the phase requires the delay request and response messages as well.
While the precision time stamping removes much of the timing uncertainty and skew within the network, packet delay variation through network elements such as switches and routers can degrade timing performance by introducing load-dependent delays and asymmetries in the forward and reverse timing paths. A method of addressing this issue is the use of transparent clocks, which are essentially switches that compensate for their own queuing delays by keeping track of the “residence time” a packet spends within the switch before being passed on. The precision time stamps in the timing packet can be updated or otherwise supplemented with the measured residence time in order to compensate for queuing delays.
However, the implementation of transparent clocks is very complex. Accurate time of day (ToD) is generally required at all of the network elements but is not always available. Time stamps corresponding to the ingress of 1588 packets must be passed through the system and must be available at the egress port for calculation. And each egress packet must be reunited with its ingress time stamp by tracing the packet through the network. Furthermore, it is often desired to retrofit PTP onto existing legacy systems that are not easily upgraded to support this capability. Accordingly, it would be useful to provide a simple system for implementing PTP transparent clocks that would overcome these challenges.